Spin injection devices

ABSTRACT

Devices such as transistors, amplifiers, frequency multipliers, and square-law detectors use injection of spin-polarized electrons from one magnetic region, into another through a control region and spin precession of injected electrons in a magnetic field induced by current in a nanowire. In one configuration, the nanowire is also one of the magnetic regions and the control region is a semiconductor region between the magnetic nanowire and the other magnetic region. Alternatively, the nanowire is insulated from the control region and the two separate magnetic regions. The relative magnetizations of the magnetic regions can be selected to achieve desired device properties. A first voltage applied between one magnetic region and the other magnetic nanowire or region causes injection of spin-polarized electrons through the control region, and a second voltage applied between the ends of the nanowire causes a current and a magnetic field that rotates electron spins to control device conductivity.

This patent document is related and hereby incorporates by reference intheir entirety U.S. patent application Ser. No. 10/284,183, filed Oct.31, 2002, entitled: “Efficient Spin-Injection Into Semiconductors” (nowU.S. Pat. No. 6,774,446); U.S. patent application Ser. No. 10/284,360,filed Oct. 31, 2002, entitled: “Magnetic Sensor Based on Efficient SpinInjection into Semiconductor” (now U.S. Pat. No. 6,809,388); co-filedU.S. patent application Ser. No. 10,632,038, entitled “Amplifiers UsingSpin Injection and Magnetic Control of Electron Spins,” (now U.S. Pat.No. 6,879,013); and co-filed U.S. patent application Ser. No.10/631,951, entitled “Square-Law Detector Based on Spin Injection andNanowires” (now U.S. Pat. No. 6,888,208).

BACKGROUND

Traditional semiconductor devices based on control of the flow and thedensity of electric charge (e.g., electrons or holes) are nearing apoint where every step towards miniaturization or towards increasing theoperating speed demands new technology and huge investments. Inparticular, as semiconductor devices become smaller (e.g., nearnanometer scale) or need to operate at faster speeds, the heat thatelectrical currents generate in semiconductor devices becomes a greaterproblem. Additionally, semiconductor devices are now reaching sizes atwhich previously ignored quantum-mechanical properties such as spin aresignificant. Dealing with these quantum-mechanical properties can be achallenge in the design of traditional semiconductor devices, but suchquantum mechanical properties also provide the potential for alternativemechanisms for device operation.

One important quantum property of electrons is their spin. The spin ofan electron gives the electron an intrinsic magnetic moment that caninteract with electromagnetic fields. The spin interactions of electronstherefore provide a potential mechanism for operational devices, andsuch devices can potentially provide much greater operating speeds andgenerate less heat than do traditional devices. The field of spintronicshas thus arisen from efforts to develop fast solid-state devices such asmagnetic sensors and transistors of nanometer proportions that use thespins or the associated magnetic moments of electrons.

S. Datta and B. Das in “Electronic Analog of the ElectroopticModulator,” Applied Physics Letters, Vol. 56, p 665 proposed a spintransistor based on the spin-orbital coupling of electrons to a gatedelectric field. Other types of spintronic devices are now sought toprovide fast operation, low heat generation, and scalability down tonanometer sizes.

SUMMARY

In accordance with an aspect of the invention, ultrafast solid-statedevices such as transistors, power current amplifiers, frequencymultipliers, and square-law detectors are based on injection ofspin-polarized electrons from a magnetic emitter to a magneticcollector. A magnetic field, which a base current through a wiregenerates in a control region between the magnetic emitter and themagnetic collector, controls rotation of the spins of injected electronsand thereby controls the conductivity of the device and the magnitude ofthe injection current. The control region can be made of a conventionalor organic semiconductor material, and the devices can be fabricatedusing integrated circuit processing techniques to generate a variety ofdevice geometries.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a perspective view of a spin injection device according toan embodiment of the invention including concentric magnetic andsemiconductor nanowires.

FIGS. 1B and 1C are cross-sectional views illustrating alternativemagnetization directions in the spin injection device of FIG. 1A.

FIGS. 2A and 2B respectively show a cross-section and a perspective viewof a spin injection device according to an embodiment of the inventionincluding magnetic and semiconductor semi-cylindrical regions formed ina trench.

FIGS. 3A and 3B respectively illustrate a cross-section and a cutawayview of a spin injection device according to an embodiment of theinvention including a semiconductor nanowire overlying a control regionthat is sandwiched between magnetic regions.

FIGS. 4A and 4B respectively show a cross-section and a cutaway view ofa device in which a nanowire overlies a semiconductor region.

FIG. 5 illustrates a spin injection device according to an embodiment ofthe invention including vertically spaced magnetic films.

Use of the same reference symbols in different figures indicates similaror identical items.

DETAILED DESCRIPTION

In accordance with an aspect of the invention, spin injection devicesinject spin-polarized electrons from a first magnetic region through anintervening control region into a second magnetic region. The devicesuse current through a nanowire to generate a magnetic field in thecontrol region, and the magnetic field in the control region rotates thespin of electrons crossing the control region. The current in thenanowire thereby controls the current through the control region bycontrolling whether the electrons that cross the control region havespins in the appropriate direction for conduction electrons entering thesecond magnetic region. Different relative orientations of themagnetizations of the first and second magnetic regions can be used togive the device properties suitable for high-speed operation as acurrent amplifier, a square-law detector, a frequency multiplier, or apulse transistor.

FIG. 1A shows a perspective view of a spin injection device 100 inaccordance with a radially-symmetric embodiment of the presentinvention. As illustrated, device 100 includes a nanowire 110 made of amagnetic material, a thin semiconductor layer 120 surrounding nanowire110, and a magnetic layer 130 surrounding semiconductor layer 120.

Magnetic nanowire 110 and magnetic layer 130 may each be formed fromvarious magnetic materials including ferromagnetic metals Ni, Fe, and Coand various magnetic alloys, which may include one or a combination ofFe, Co, Ni, CrO₂, and Fe₃O₄ and different magnetic semiconductors suchas GaAs:Mn, CaB₆, and Ca_(1-x)La_(x)B₆.

Semiconductor layer 120 may be formed from various semiconductormaterials including Si, Ge, GaAs, ZnTe, GaSb, GaP, InAs, CdSe, InP,InSb, CdTe, CdS, ZnS, ZnSe, AlP, AlAs, AlSb and also alloys andcombinations of these materials. For reasons discussed below,semiconductor layer 120 is preferably formed from a material such asGaAs, GaInAs, Ge, Si, ZnSe and ZnCdSe that provides a relatively largetime τ_(S) of electron spin relaxation and preferably has a negativedoping.

Operation of device 100 is based on injection of spin-polarizedelectrons from magnetic nanowire 110 into magnetic layer 130 throughsemiconductor layer 120. Generally, for alternative embodiments of theinvention described further below, an angle θ₀ between a magnetizationM₁ of nanowire 110 and a magnetization M₂ of magnetic layer 130 can beselected as required to provide device 100 with desired properties. FIG.1B illustrates an exemplary embodiment in which magnetic nanowire 110has a magnetization M₁ directed along the axis of nanowire 110, andmagnetic layer 130 has a magnetization M₂ that is opposite (orantiparallel) to magnetization M₁. FIG. 1C illustrates one alternativeembodiment where magnetic nanowire 110 has a magnetization M₁ directedalong the axis of nanowire 110, and magnetic layer 130 has amagnetization M₂ that is perpendicular to magnetization M₁. As a result,the angle θ₀ between magnetizations M₁ and M₂ is 180° or π for theconfiguration of FIG. 1B and is 90° or π/2 for the configuration of FIG.1C. A flow of electrons from magnetic nanowire 110 to magnetic layer 130generally depends on the angle θ₀ between the magnetizations in magneticlayers 110 and 130, and on the rotation of electron spins insemiconductor layer 120.

FIG. 1A shows electrical contacts 140 and 150 at opposite ends ofnanowire 110. In operation, a base voltage V_(b) applied betweenelectrical contacts 140 and 150 causes a base current J_(b) to flowthrough magnetic nanowire 110. As a definite example, the followingdiscussion assumes that contact 140 is grounded and that base voltageV_(b) applied to terminal 150 has a negative polarity, causing a currentflow from contact 140 to 150 (i.e., into the page in FIG. 1B or 1C). Abase voltage V_(b) of the positive polarity could alternatively be used.Base current J_(b) through magnetic nanowire 110 creates insemiconductor layer 120 a magnetic field H that is tangential toconcentric circles centered on nanowire 110, and magnetic field H isclockwise in FIGS. 1B and 1C for the specific example of a currentflowing from contact 140 to contact 150. Static magnetic fields createdby magnetic regions 110 and 130 are effectively zero everywhere with theexception of ends of semiconductor layer 120. This edge effect isnegligible in semiconductor layer 120 since semiconductor layer 120 maya length about or larger than 1 μm, which is much more than its typicalthickness of 10 to 100 nm.

For electron injection, an emitter voltage V_(e) is applied betweenelectrical contacts 160 and 140. As a result, a current may flow throughsemiconductor layer 120 between magnetic nanowire 110 and magnetic layer130. As a specific example, the following description assumes thatemitter voltage V_(e) has a positive polarity so that the direction ofelectrons flow is from magnetic nanowire 110 to magnetic layer 130.Alternatively, device 100 could use an emitter voltage V_(e) with anegative polarity, which would cause an electron flow in the oppositedirection.

FIGS. 1B and 1C show the directions of the drift velocity v and spin σof electrons entering semiconductor layer 120 from magnetic nanowire 110when emitter voltage V_(e) is positive. The amount of current thatactually flows between magnetic nanowire 110 and magnetic layer 130depends on whether magnetic field H inside semiconductor layer 120rotates of spin σ toward or away from the predominant direction ofconduction electrons in magnetic layer 130.

When magnetic nanowire 110 injects spin-polarized electrons intomagnetic layer 130 through semiconductor layer 120, a transit time τ_(T)of the electrons traversing semiconductor layer 120 should not exceedthe spin-relaxation time τ_(S), i.e., the time of spin coherence ofelectrons in semiconductor layer 120. In other words, spin ballistictransport is desired. Theoretical calculations and experimental studiesindicate that the longest values for spin-relaxation time τ_(S) can berealized in negatively-doped (i.e., n-type) semiconductors and can reachup to 1 ns in materials such as n-ZnSe and n-GaAs at room temperature.

Transit time τ_(T) is equal to the ratio of the thickness d ofsemiconductor layer 120 and the drift velocity v of the electronstraversing semiconductor layer 120. The drift velocity v in turn dependson the applied electric field E, the electron mobility μ_(n), and thediffusion constant D_(n) in semiconductor layer 120 as indicated inEquation (1). From Equation (1), the condition that transit time τ_(T)be less than spin-relaxation time τ_(S) limits the maximum thicknessd_(max) of semiconductor layer 120 for spin ballistic transport asindicated in Equation (2). The maximum thickness d_(max) for a typicalsemiconductor with suitable characteristics is generally about or lessthan 1 μm.v=μ _(n) Ē+D _(n) /d  (1)τ_(t)≦τ_(s), or d<d _(max) =√{square root over (D_(n)τ_(S))}+τ _(S)μ_(n)E  (2)

Transit time τ_(T) and the corresponding angular frequency ω₀ or2π/τ_(T) are also limits on the operating speed or frequency of device100. However, with transit time τ_(T) less than a 1-ns spin-relaxationtime τ_(S), device 100 can operate at a frequency of 1 GHz or more.

The conductivity G between concentric magnetic layers 110 and 130, i.e.,between electrical contacts 140 and 160 generally depends on an angle θbetween spin directions of electrons in semiconductor layer 120 near themagnetic layer 130 and the magnetization in the magnetic layer 130. Thereceiving interface effectively acts as a spin filter with a resistancedepending on spin orientation of the spin-polarized electrons in thecontrol region 120, near the receiving interface.

Equation (3) indicates the conductivity G between magnetic layers 110and 130 when semiconductor layer 120 is sufficiently thin that the spinpolarization states of the electrons traversing layer semiconductor 120remain coherent. In particular, Equation (3) is valid when semiconductorlayer 120 is relatively thick and the spin ballistic transport isrealized, i.e., the condition of Equation (2) is fulfilled. In Equation(3), parameters P₁ and P₂ represent the degrees of spin polarizations ofcurrents crossing the first and second magnetic-semiconductor (M-S)interfaces. Thus, parameter P₁ represents the spin polarization ofelectrons entering semiconductor layer 120 from magnetic nanowire 110,and parameter P₂ represents the spin polarization of the spin-polarizedelectrons entering semiconductor layer 120 from magnetic layer 130.Angle θ is the angle between the spin σ of injected spin-polarizedelectrons in semiconductor layer 120 near the magnetic layer 130 and themagnetization M₂ in the magnetic layer 130.G=G ₀(1+P ₁ P ₂ cos θ)=G ₀[1+P ₁ P ₂ cos(θ₀+θ_(H))]  (3)

Magnetization M₁ in magnetic nanowire 110 determines the direction ofspin-polarized conduction electrons injected into semiconductor layer120 from the magnetic nanowire 110. Absent spin rotation insemiconductor layer 120, angle θ is equal to the angle θ₀ betweenmagnetizations M₁ and M₂, which is 180° for the magnetization directionsillustrated in FIG. 1B and 90° for the magnetization directionsillustrated in FIG. 1C. The illustrated embodiment of FIG. 1B thusprovides a minimum conductivity G₀(1−P₁P₂) if the electrons retain theirspin when crossing semiconductor layer 120. However, magnetic field Hcan be used to change the spin directions and therefore change theconductivity G between contacts 140 and 150. When accounting for arotation of the spins of electrons traversing semiconductor layer 120,angle θ is equal to the sum θ₀+θ_(H) where angle θ₀ is the angle betweenthe magnetizations M₁ and M₂ of magnetic materials 110 and 130 and angleθ_(H) is the amount of spin rotation in semiconductor layer 120.

Rotation angle θ_(H) depends on magnetic field H in semiconductor layer120. In particular, base voltage V_(b) drives base current J_(b) thatflows along magnetic nanowire 110 and induces a radially symmetricalmagnetic field H around magnetic nanowire 110. Equation (4) indicatesthe magnitude of magnetic field H in terms of base current J_(b) and aradial distance ρ from the center of nanowire 110. When base currentJ_(b) is greater than 25 mA and radius ρ in semiconductor layer 120 isless than about 40 nm, the magnitude of magnetic field H will be greaterthan about 1000 Oe. A very thin wire, i.e., a nanowire 110 having aradius less than about 40 nm can thus provide a strong enough magneticfield.H=J _(b)/2πρ  (4)

The spins σ of the injected spin-polarized electrons precess in magneticfield H during transit through semiconductor layer 120. The spinrotation of an electron in a magnetic field H is well known to have anangular frequency γH_(n) where the electron gyromagnetic ratio in vacuumγ is about 1.76×10⁷Oe⁻¹s⁻¹ or 2.2×10⁵ (m/A)s⁻¹ and field component H_(n)is the magnetic field component normal to the spin. In device 100,magnetic field H remains perpendicular to the spin direction, andcomponent H_(n) is equal to the magnitude of magnetic field H. Therotation angle θ_(H) for the spin of an electron crossing semiconductorlayer 120 is thus approximately given in Equations (5), where a variablek_(j) is a gain factor, which is introduced to simplify Equation (5).The factor g below is the gyromagnetic factor, which is close to 2 invacuum but may have a different value in the solid state matrix ofsemiconductor layer 120.

$\begin{matrix}{\begin{matrix}{{\theta_{H} = {{\frac{g}{2}{\gamma\tau}_{t}H} = {{\frac{g}{2}{\gamma\tau}_{t}{{J_{b}(t)}/2}{\pi\rho}} \equiv {k_{J}{J_{b}(t)}}}}},} \\{k_{J} \equiv {g\;{{\gamma\tau}_{t}/4}{\pi\rho}}}\end{matrix},} & (5)\end{matrix}$

The conductivity of device 100 as indicated in Equation (3) above thusdepends on the base current J_(b) and on the directions ofmagnetizations M₁ and M₂. Base current J_(b) can be static ortime-dependent, e.g., of the form J_(b0) cos(ωt) for an angularfrequency ω that is less than 2π/τ_(t). Magnetizations M₁ and M₂ ofmagnetic wire 110 and magnetic layer 130, respectively, have directionsthat can be selected to achieve desired device characteristics. Inparticular, different relative orientations of magnetizations M₁ and M₂provide devices capable of acting as a current amplifier, a square lawdetector, a frequency multiplier, or a pulse transistor.

Device 100 of FIG. 1A can operate as a current amplifier when themagnetization M₁ of magnetic nanowire 110 is substantially perpendicularto the magnetization M₂ of magnetic layer 130. As a rule, themagnetization M₁ of a thin ferromagnetic wire 110 is directed along anaxis of the wire. Magnetization M₂ can be made tangential to the surfaceof magnetic layer 130, as illustrated in FIG. 1C, when magnetic layer130 has a thickness d less than a domain wall thickness L₀ of themagnetic material but greater than about 3–5 mm. Alternatively,magnetization M₂ of magnetic layer 130 can be made perpendicular to thesurface of the ferromagnetic film when magnetic layer 130 has athickness d that is equal to or less than about 4–5 nm. In either case,angle θ₀ between magnetizations M₁ and M₂ is equal to π/2 (i.e., a rightangle), and Equation (3) simplifies to the form of Equation (6).G=G ₀(1+P ₁ P ₂ sin θ_(H))  (6)

When a constant emitter voltage V_(e) is applied, the “emitter” currentJ_(e) through device 100 is equal to the product of emitter voltageV_(e) and the conductivity G. For a small rotation angle θ_(H), Equation(7) indicates that the variable component emitter current J_(e) isproportional to rotation angles θ_(H) and therefore to base currentJ_(b). In Equation (7), the current J_(e0) is equal to the product ofconductivity constant G₀ and emitter voltage V_(e), parameters P₁ and P₂are the degrees of spin polarization at the semiconductor-magneticinterfaces, k_(j) is the variable introduced for Equation (5) evaluatedat a radial distance ρ_(s) that is a typical radius of semiconductorlayer 120 (i.e., k_(J)=gγτ_(t)/4πρ_(s)). The current gain K_(J) of theamplifier is indicated in Equation (8).J _(e) =GV _(e) ≈J _(e0)(1+P ₁ P ₂θ_(H))=J _(e0)(1+P ₁ P ₂ k _(J) J_(b))  (7)K _(J) =∂J _(e) /∂J _(b) =J _(e0) P ₁ P ₂ k _(J)  (8)

Equation (7) and particularly the dependence of gain factor k_(J) onradial distance ρ indicates that a very thin wire 110, i.e., with theradius ρ₀ less than 100 nm can provide a larger current gain. Indeed,gain factor k_(J) is equal to 10³ A⁻¹ when semiconductor radius ρ_(s) isequal to about 30 nm and transit time τ_(T) greater than 10⁻¹⁰ s, andcurrent gain K_(J) can amount to very large value for ultrahigh angularfrequency 2π/τ_(t) greater than 100 GHz even when the product of spinfactors P₁P₂ is less than 0.1.

Device 100 can implement an ultrafast square law detector or frequencymultiplier when the magnetization M₁ inside magnetic nanowire 110 issubstantially parallel or anti-parallel to the magnetization M₂ insidemagnetic layer 130. Parallel or antiparallel magnetizations can berealized when magnetic nanowire 110 is relatively thin (less than about4–5 nm) and the thickness d of magnetic layer 130 is greater than 5 nmbut less that the typical width L of a magnetic domain wall in magneticlayer 130. In these cases, angle θ₀ is 0 or π for the parallel orantiparallel magnetizations, and a combination of Equations (3) and (5)indicates that for small angles θ_(H) and a constant emitter voltageV_(e), the emitter current J_(e) depends on the base current J_(b) asindicated in Equation (9).J _(e) =J _(e0)(1±P ₁ P ₂ cosθ_(H))≈J _(e0)(1±P ₁ P ₂)∓J _(e0) P ₁ P ₂ k_(J) ² J _(b) ²(t)   (9)

The case when magnetizations M₁ and M₂ are antiparallel, as illustratedin FIG. 1B, is preferable because the constant term J_(e0)(1−P₁P₂) forthe antiparallel magnetizations is less than the constant termJ_(e0)(1+P₁P₂) for the parallel magnetizations. Equation (9) shows thatthe time-dependent component J_(e0)P₁P₂k_(J) ²J_(b) ²(t) of emittercurrent J_(e) is proportional to the square of base current J_(b). Asquare-law detector thus generates the emitter current J_(e) as a signalwith changes that are proportional to the square of an input signalJ_(b).

When base current J_(b)(t) has a sinusoidal time dependence, e.g., basecurrent J_(b)(t) is proportional to cos(ωt), the emitter current J_(e)has a time-dependent component that varies as the square of thesinusoid, e.g., a component proportional to cos²(ωt)=½[1+cos(2ωt)] andtherefore includes a component that varies at twice the frequency ofbase current J_(b). The device thus provides a doubling of frequency,which can be realized up to ultrahigh frequencies, e.g., 500 GHz.Squared variable k_(J) ² is about equal to 10⁶ A⁻² when semiconductorradius ρ_(s) is less than about 30 nm and transit time τ_(T) is greaterthan or equal to 10⁻¹⁰ s.

When base current J_(b) is a superposition of components having twodifferent frequencies ω₁ and ω₂, e.g., J_(b)(t)∝cos(ω₁t)+cos(ω₂t), theemitter current J_(e) for fixed emitter voltage V_(e) includescomponents that vary at twice the individual frequencies 2ω₁ and 2ω₂, atthe sum ω₁+ω₂ of the individual frequencies, and at the difference ω₁−ω₂of the individual frequencies. The component having the desiredfrequency can be extracted using a resonance amplifier for the desiredfrequency. The emitter current is then able to work as a heterodyneoscillator to generate and amplify a signal having doubled frequency 2ω₁or 2ω₂, the sum frequency ω₁+ω₂, or the difference frequency ω₁−ω₂.

A high-speed transistor can be realized when the magnetization M₁ ofmagnetic nanowire 110 is antiparallel to the magnetization M₂ ofmagnetic layer 130. In this case, angle θ₀ is equal to π, and Equation(10) indicates the emitter current J_(e) for a constant applied emittervoltage V_(e).J _(e) =GV _(e) =G ₀ V _(e)(1−P ₁ P ₂ cos θ_(H))=G ₀ V _(e)(1−P ₁ P ₂cos(k _(J)J_(b)))  (10)

Emitter current J_(e) of Equation (10) reaches a maximum emitter currentJ_(emax) when θ_(H)=k_(J)J_(b) is equal to π. Variable k_(J), as notedabove, has a value of about 10³ A⁻¹ when radius ρ_(s) is equal to about30 nm and τ_(t) is equal to or greater than 10⁻¹⁰ s, so that rotationangle θ_(H) will be equal to π for a base current J_(b) close to 1 mA.In other words, when base current J_(b) pulses with an amplitude ofabout 1 mA for a duration greater than transit time τ_(t), the variablecomponent of emitter current J_(e) increases by a factor K_(m), which isgiven in Equation (11). If the spin polarization fractions P₁ and P₂ areclose to unity, the variation of the emitter current J_(e) can reachseveral orders of magnitude.J _(emax) /J _(emin)=(1+P ₁ P ₂)/(1−P ₁ P ₂)=K _(m)  (11)

Another transistor configuration has the magnetization M₁ in magneticnanowire 110 perpendicular to the magnetization M₂ in magnetic layer130. From Equations (6) and (7), the time-dependent component of emittercurrent J_(e) for a constant emitter voltage V_(e) can be shown to havethe form of Equation (12) when emitter current J_(e) is relativelysmall. Since gain factor k_(J) is about 10³ A⁻¹ when radius ρ_(s) isequal to about 30 nm and τ_(T) is equal to or greater than 10⁻¹⁰ s, theamplitude of time dependent component J_(e)(t) of the emitter currentcan thus exceed the amplitude of base current J_(b)(t) by several ordersof amplitude even when the product P₁P₂ of the spin polarizationfractions is much less than 1. In other words, the spin-injectiontransistor may be a sensitive device even for a short pulse durationgreater than transit time τ_(t). For a relatively small base currentJ_(B)<<k_(J), emitter current J_(e) is approximately given by Equation(12).J _(e)(t)≈J _(e0) P ₁ P ₂θ_(H) =P ₁ P ₂ k _(J) J _(e0) J _(b)(t)  (12)

Device 100 of FIGS. 1A, 1B, and 1C is useful for illustration of some ofthe principles of the invention but is difficult to construct.Alternative embodiments of the invention can be constructed using acombination of fabrication processes that are known for creatingmagnetic layers and semiconductor devices. FIGS. 2A and 2B, for example,are an end view and a cutaway perspective view of spin injection device200 in accordance with an embodiment of the invention that is moreeasily manufactured. Device 200 of FIGS. 2A and 2B uses magnetic andsemiconductor layers that can be operated in the manner described abovefor device 100 of FIGS. 1A, 1B and 1C to implement devices such ascurrent amplifiers, square law detectors, frequency multipliers, andpulse transistors.

FIGS. 2A and 2B respectively illustrate an end view and a perspectiveview of device 200. As illustrated, device 200 includes a nanowire 210,a dielectric layer 215, a first magnetic layer (emitter) 220, asemiconductor (control) layer 230, and a second magnetic layer(collector) 240 that are formed in and on a substrate 290. The device200 further includes electrodes 250 and 260 that respectively contactmagnetic layers 220 and 240, and electrodes 270 and 280 that contactopposite ends of nanowire 210.

Nanowire 210 is preferably a metal wire having a semicircularcross-section and a radius ρ₀ less than about 100 nm. The thickness ofthe other layers 215, 220, 230 and 240 are less than radius ρ₀, with thethickness w of dielectric layer 215 preferably being less than 10 or 20nm and more than 1 or 2 nm (the latter being needed in to reduce aleakage current). Device 200 differs from device 100 in that dielectriclayer 215 isolates the current carrying nanowire 210 from the firstmagnetic layer 220, so that a magnetic material is not required toconduct the base current J_(b) that generates the magnetic field H insemiconductor layer 230. Nanowire 210 can thus be made of a highlyconductive metal such as Al, Au, Pt, Ag, or Cu or polysilicon or ahighly doped semiconductor such as Si, GaAs, InSb, InAs, InGa, or InP. Avoltage drop in base circuit J_(b) between electrodes 270 and 280 canthus be less in device 200 than in device 100 of FIGS. 1A and 1B.Additionally, the bias voltage V_(b) driving the base current J_(b) isseparate from and has less effect on the emitter circuit, i.e., betweenelectrodes 250 and 260.

A fabrication process for device 200 begins with substrate 290, whichcan be a semiconductor, including Si, or dielectric substrate, in whicha trench has been etched or otherwise formed. The trench is preferablysemicircular with a radius equal to the outer radius of magnetic layer240, e.g., about 30 nm to 150 nm. Alternatively, a trench having arectangular or other shape could be employed.

Magnetic material is then deposited on substrate 290 and particularly inthe trench in a manner that provides the desired magnetization directionM₂ for magnetic layer 240. One deposition method for magnetic layer 240deposits a thin conformal layer having a thickness between about 3 nmand 30 nm on substrate 290. Alternatively, a thicker magnetic layer canbe deposited and etched back to the desired dimensions of magnetic layer240. An anti-ferromagnetic pinning layer made of a material such asFeMn, IrMn, NiO, MnPt (Ll₀), or α-Fe₂O₃ can be provided under magneticlayer 240 to pin the direction of magnetization M₂.

Semiconductor layer 230, which can be a material such as Si, Ge, ZnSe orGaAs having a relatively long spin coherence time τ_(s), is thendeposited over magnetic layer 240. Again, semiconductor layer 230 caneither be deposited conformally to the desired thickness, e.g., about 20to 100 nm, or etched after deposition of a thicker layer to leavesemiconductor layer 230 and the remaining trench with their desiredsizes. Semiconductor layer 230 is preferably an n-type material and caneither include an in-situ n-type doping when deposited or can be dopedwith appropriate impurities after deposition.

Magnetic layer 220 is formed on semiconductor layer 230 using the sametechniques as used to form magnetic layer 240 with the exception thatthe magnetization M₁ of magnetic layer 220 may differ from themagnetization M₂ of magnetic layer 240.

Dielectric layer 215 can then be deposited or grown on magnetic layer220. Dielectric layer 215 can be a material such as SiO₂, Al₂O₃, orother metal oxide and preferably has a thickness between about 2 nm and5 nm. After formation of dielectric layer 215, a deposition of a highconductivity metal fills the remaining portion of the trench and formsnanowire 210.

After formation of magnetic layer 240, semiconductor layer 230, magneticlayer 220, insulating layer 215, and nanowire 210, the top surface ofsubstrate 290 can be planarized or etched to remove the depositedmaterials from the portion of the surface of substrate 290 that isoutside the trench. Electrodes 250, 260, 270, and 280 can then be formedto contact respective layers 220, 240, and 210 using conventionaltechniques. Electrode 260 contacting magnetic layer 240 canalternatively be formed in substrate 290 before formation of the trench.

In the illustrated embodiment, electrode 260 contacts the outer magneticlayer 240 and can be formed by patterning a metal layer depositeddirectly on substrate 290. Electrode 250, which contacts inner magneticlayer 220, is formed on an insulating layer 245 including an openingcontaining a conductive plug 255. Conductive plug 255 can optionally bemade of the same ferromagnetic material and with the same magnetizationM₁ as magnetic layer 220 to minimize injection of electrons that are notspin-polarized where plug 255 is near or overlapping semiconductor layer220. Electrodes 250 and 260 can contact respective magnetic regions 220and 240 along their entire length of magnetic regions 240 and 250, sothat the conductivity of the emitter circuit in device 200 is high.

FIG. 2B also illustrates that end contacts 270 and 280 to nanowire 210can be formed away from layers 220, 230, and 240 by extending nanowire210 further than the lengths of layers 220, 230, and 240. This can beachieved by limiting the lateral extent of layers 220, 230, and 240 orby extending the length of the trench after formation of magnetic layer220, so that deposited nanowire 210 extends beyond layers 220, 230, and240.

FIGS. 3A and 3B show a spin injection device 300 in accordance with anembodiment of the invention that can be fabricated without forminglayers in a trench. Device 300 includes a conductive nanowire 310 thatis nearly semicircular and covered by a thin dielectric layer 315, afirst magnetic layer 320, a semiconductor layer 330, and a secondmagnetic layer 340. Electrodes 350 and 360 contact magnetic layers 320and 340, and electrodes 370 and 380 contact the face planes (or ends) ofnanowire 310.

Nanowire 310 has radius ρ₀ of that obviously less than the radius ρ_(s)of semiconductor layer 330, which is preferably less than 100 nm. Thelength of nanowire 310 can be on the order of about 1 μm but is moregenerally limited by acceptable maximum resistance of nanowire 310.

The thickness dielectric layer 315 between nanowire 310 and magneticlayer 320 is preferably greater than 1 to 2 nm to isolate nanowire 310from magnetic layer 320 but is preferably less than 10 to 20 nm. Sincedielectric layer 315 isolates nanowire 310 from magnetic layer 320, theelectrical circuits for base current J_(b) and emitter current J_(e) areindependent. The conductivity G of the emitter circuit in device 300 ismuch higher than the conductivity of the base circuit merely fromgeometry (i.e., the areas) of layers 320, 330, and 340 in device 300.

Magnetic layers 320 and 340 have fixed magnetizations M₁ and M₂ that areselected according to the desired properties of device 300. Thethickness d₁ of magnetic layer 320 should be less than radius ρ_(S) andis preferably less than a typical width L₀ of magnetic domain walls inthe magnetic material. Usually, domain wall width L₀ is about 10–50 nm.When magnetic layer 320 has a thickness d₁ less than the width L₀ ofmagnetic domain walls but greater than 3 to 5 nm, the magnetization M₁of thin magnetic film 320 lies in the film plane and can be directedalong the axis of nanowire 310.

The thickness d of semiconductor layer 330 between magnetic layers 320and 340 is preferably greater than 10 nm but less than 100 nm.

In a fabrication process for device 300, a substrate 390 is prepared tocontain underlying conductive contacts such as part of electrodes 360,370, and 380. The underlying contacts can be made of a conductivematerial such as a metal or a highly doped region of semiconductormaterial. Regions of insulating dielectric 315 can be provided insubstrate 390 where required to isolate the conductive structures.

A layer of highly conductive material is then deposited on substrate 390with a pattern that forms nanowire 310 in an area isolated from thecontacts in substrate 390. The patterned material can be made by pressforming and can be heated or otherwise liquefied so that beadingprovides the desired semicircular cross-section to nanowire 310. U.S.Pat. No. 6,432,740, which is incorporated by reference in its entirety,describes a suitable method for forming a nanowire of the appropriatesize, but other methods could alternatively be used.

Additional portions of thin dielectric layer 315 are then grown and/ordeposited on nanowire 310. Magnetic layer 320 is deposited to overliedielectric layer 315 and nanowire 310 and to extend laterally far enoughto make good contact with underlying terminal 360.

A portion of an insulating layer 345 is formed on terminal 360 beforesemiconductor layer 330 is deposited on magnetic layer 320, and amagnetic layer 340 is formed on semiconductor layer 330. Insulatinglayer 345 electrically insulates magnetic layer 320 and semiconductorlayer 330 from terminal 360. A metal layer or other conductive layerforming terminal 350 is deposited in electrical contact with magneticlayer 340. The remainder of insulating dielectric layer 345 can then bedeposited to provide insulation with openings for electrical connectionsto terminals 350, 360, 370, and 380.

FIGS. 4A and 4B illustrate a device 400 in which a nanowire 410 overliesa semiconductor region. FIG. 4A shows a cross-section of device 400, andFIG. 4B shows a cutaway view of device 400 after removal of a topportion designated by the section line 4B in FIG. 4A.

As illustrated, device 400 includes nanowire 410, a dielectric layer415, semiconductor region 420, a first magnetic region 430, and a secondmagnetic region 440. Magnetic regions 430 and 440 and semiconductorregion 420, which is between magnetic regions 430 and 440, are on asurface of a substrate 490. Electrodes 460 and 470, which respectivelycontact magnetic regions 430 and 440, extend along the surface ofsubstrate 490 to external contacts (not shown) or to other devices (notshown) that may be formed on substrate 490. Regions 420, 430, and 440and electrodes 460 and 470 can be formed in a series of deposition andpatterning processes using photolithographic patterning techniques.Alternatively, the patterned materials can be made by press forming suchas described in U.S. Pat. No. 6,432,740, which is incorporated byreference in its entirety. The resulting structure can be madesubstantially planar with the exception of an optional indentation ortrench in semiconductor region 420 that may be formed to accommodatenanowire 410.

A layer of an insulating material such as SiO₂, Al₂O₃ or other metaloxides about 2 to 4 nm thick is deposited on regions 420, 430, and 440and electrodes 460 and 470 to form a first portion of dielectric layer415. Nanowire 410, which can be made of a high conductivity metal, isthen formed, for example, by metal deposition and patterning.Optionally, electrodes 450 and 480 contacting opposite ends of nanowire410 can be formed at this time from the same material as nanowire 410.After formation of nanowire 410, a second portion of dielectric layer415 is deposited to encircle and insulate nanowire 410. If electrodes450 and 480 were not previously formed, electrodes 450 and 480 can beformed on dielectric layer 415 and contact the ends of nanowire 410through openings in dielectric layer 415.

Semiconductor region 420 should have a thickness less than about 100 nmto maintain the coherence of the spin states of electrons traversingsemiconductor region 420. The typical thickness or diameter of nanowire410 should also be less than about 100 nm to provide a strong magneticfield H in semiconductor region 420. The thickness of magnetic layers430 and 440 is approximately the same as the thickness of semiconductorlayer 420 in FIG. 4A but may differ in other device configurations.

Dielectric layer 415 should have a thickness between metal nanowire 410and semiconductor layer 420 that is less than the radius ρ₀ of nanowire410 to provide a strong magnetic field but thicker than about 2 nm toprovide adequate insulation. The dielectric layer 415 isolates nanowire410 from semiconductor layer 420 and magnetic layers 430 and 440.

Nanowire 410, being separated from the emitter circuit by dielectriclayer 415, can be made from a high conductive metal such as Al, Au, Pt,Ag, or Cu or polysilicon or highly doped semiconductor such as Si, GaAs,InSb, InAs, InGa, or InP. As a result, the drop in base voltage V_(b)between electrodes 450 and 480 is less than the base voltage drop fordevice 100 of FIG. 1A, and base voltage V_(b) does not affect thevoltage drop in the emitter circuit, i.e., between electrodes 460 and470. Moreover, the conductivity of the emitter circuit is much highermerely from the geometry of device 400. In particular, nanowire 410 mayhave a diameter that is a few tens of nanometers, while magnetic regions430 and 440 have thicknesses of a few tens of nanometers but lengthstypically on the order of 1 μm or larger.

FIG. 5 shows a spin injection device 500 according to an embodiment ofthe invention using vertically-spaced magnetic regions 530 and 540.Device 500 includes a semiconductor region 520 on magnetic region 530and a nanowire 510 that overlies semiconductor region 520. Magneticregions 540 overlap opposite ends of semiconductor region 520. Adielectric layer 515, which includes a portion that separates nanowire510 from magnetic regions 540 and semiconductor region 520, is shown asbeing transparent in FIG. 5 to better illustrate underlying structures.

Device 500 can be fabricated by first forming a conductive interconnectregion 560 (e.g., a metal or highly doped semiconductor region) in or ona dielectric or semiconductor substrate 590. Interconnect region 560provides an electrical connection to magnetic region 530, which isdeposited on interconnect region 560. An external contact (not shown) orconnection to another device (not shown) on substrate 590 can belaterally separated from device 500. Semiconductor region 520 isdeposited on magnetic region 530 and can be patterned using the samemask that controls the dimensions of magnetic region 530. Dielectricregions 535 are then deposited around magnetic region 530 andsemiconductor region 520.

Magnetic layer 540 can be deposited on semiconductor region 520 andadjacent dielectric regions 535. Magnetic layers 540, like magneticlayer 530, can be formed of a ferromagnetic material or other materialthat has spin-polarized conduction electrons. Patterning of magneticlayer 540 exposes a central portion of semiconductor region 520 butleaves magnetic regions 540 in contact with outer portions ofsemiconductor region 520. Optionally, an etching process through theopening between magnetic regions 540 can create a depression insemiconductor region 520. A thin insulating layer and nanowire 510 arethen formed in the opening between magnetic regions 540. The insulatinglayer becomes part of dielectric layer 515, which separates nanowire 510from the emitter circuit and therefore permits use of highly conductivemetals such as Al, Au, Pt, Ag and Cu or polysilicon or highly dopedsemiconductor such as Si, GaAs, InSb, InAs, InGa, and InP for nanowire510.

Contacts to nanowire 510, magnetic region 530, and magnetic regions 540can then be formed. In particular, metal contact 570 can be formed onmagnetic regions 540 and covered with the remainder of dielectric layer515. Openings can then be formed through dielectric layer 515 forcontacts 550 and 580 to the opposite ends of nanowire 510.

The typical sizes of cross-sections of metal nanowire 510 andsemiconductor layer 520 should be less than about 100 nm, and thethickness of magnetic layers 530 and 540 may be close to the thicknessof semiconductor layer 520. The portion of dielectric layer 515 betweenmetal nanowire 510 and semiconductor layer 520 should be greater thanabout 2 nm but less than about 100 nm.

Although the invention has been described with reference to particularembodiments, the description is only an example of the invention'sapplication and should not be taken as a limitation. Many additions orvariations can be applied in the disclosed amplifiers. For example,anti-ferromagnetic layers made of a material such as FeMn, IrMn, NiO,MnPt (Ll₀), or α-Fe₂O₃ can be added to fix the directions ofmagnetizations M₁ and M₂ in the magnetic films.

Additionally, when ferromagnetic metals Ni, Fe, and/or Co are used asmagnetic layers in the structures described above, so-called δ-dopedlayers, which are semiconductor layers that are extremely thin and veryheavily doped with n-type dopants, may be formed between semiconductorand magnetic layers, i.e., the δ-doped layers may be located inside theferromagnetic-semiconductor junction. The specifications of the δ-dopedlayers are formulated in U.S. patent application Ser. No. 10/284,183,filed Oct. 31, 2002, entitled: “Efficient Spin-Injection IntoSemiconductors” and U.S. patent application Ser. No. 10/284,360, filedOct. 31, 2002, entitled: “Magnetic Sensor Based on Efficient SpinInjection into Semiconductor”.

Various other adaptations and combinations of features of theembodiments disclosed are within the scope of the invention as definedby the following claims.

1. A device comprising: a first magnetic region; a second magneticregion; a control region that forms a first interface with the firstmagnetic region and a second interface with the second magnetic region;and a wire positioned relative to the control region so that a currentthrough the wire creates in the control region a magnetic field thatrotates spins of the electrons traversing the control region.
 2. Thedevice of claim 1, wherein the control region is such that an electronspin relaxation time of the control region is longer than a transit timeof the electrons traversing control region.
 3. The device of claim 1,wherein the control region comprises a semiconductor material.
 4. Thedevice of claim 3, wherein the semiconductor material is selected from agroup consisting of Si, Ge, GaAs, InAs, GaP, GaInAs, ZnSe, and ZnCdSe.5. The device of claim 3, wherein the semiconductor material is n-type.6. The device of claim 1, wherein each of the first and second magneticregions comprises a ferromagnetic material.
 7. The device of claim 1,wherein the first magnetic region has a first magnetization, the secondmagnetic region has a second magnetization, and the first and secondmagnetizations are fixed at a relative angle selected to give the devicea desired electrical characteristic.
 8. The device of claim 1, furthercomprising terminals that permit biasing of the first and secondmagnetic regions to cause injection of spin-polarized electrons throughthe first interface into the control region so that the second interfaceacts as a spin filter with a resistance depending on spin orientation ofthe spin-polarized electrons in the control region, near the secondinterface.
 9. The device of claim 1, wherein a bias voltage appliedbetween the first and second magnetic regions causes injection ofspin-polarized electrons through the control region between the firstmagnetic region and the second magnetic region.
 10. The device of claim1, wherein a fixed bias voltage is applied between the first and secondmagnetic regions, and a first current through the wire changes a secondcurrent between the first and second magnetic regions.
 11. The device ofclaim 1, further comprising an insulating material disposed toelectrically insulate the wire from the control region, the firstmagnetic region, and the second magnetic region.
 12. A devicecomprising: a magnetic wire; a magnetic region; and a control regionforming a first interface with the magnetic wire and a second interfacewith the magnetic region, wherein: the first and second interfacesselectively permit spin-polarized electrons to cross between themagnetic wire and the magnetic region; and a current along the magneticwire creates in the control region a magnetic field that rotates spinsof the electrons traversing the control region to flow between themagnetic region and the magnetic wire.
 13. The device of claim 12,wherein the control region is such that an electron spin relaxation timeof the control region is longer than a transit time of the electronstraversing control region.
 14. The device of claim 12, wherein thecontrol region comprises a semiconductor material.
 15. The device ofclaim 14, wherein the semiconductor material is selected from a groupconsisting of Si, Ge, GaAs, InAs, InP, GaInAs, ZnSe, and ZnCdSe.
 16. Thedevice of claim 14, wherein the semiconductor material is n-type. 17.The device of claim 12, wherein the magnetic wire comprises aferromagnetic material.
 18. The device of claim 12, wherein the magneticregion comprises a ferromagnetic material.
 19. The device of claim 12,wherein the magnetic wire has a first magnetization, the magnetic regionhas a second magnetization, and the first and second magnetizations arefixed at a relative angle selected to give the device a desiredelectrical characteristic.
 20. The device of claim 12, wherein a biasvoltage applied between the magnetic wire and the magnetic region causesinjection of spin-polarized electrons through the control region betweenthe magnetic wire and the magnetic region.
 21. The device of claim 12,wherein a fixed bias voltage is applied between the magnetic wire andthe magnetic region, and a first current through the magnetic wirechanges a second current between the magnetic wire and the magneticregion.
 22. The device of claim 12, further comprising a first andsecond contacts connected to opposite ends of the magnetic wire, whereinthe current along the wire flows between the first and second contacts.23. The device of claim 1, further comprising a first and secondcontacts connected to opposite ends of the wire, wherein the currentcreating the magnetic field flows between the first and second contacts.